A highly-unsaturated fatty acid is a fatty acid having two or more unsaturated bonds and includes ω6 unsaturated fatty acids such as linoleic acid (LA, 18: 2n−6), γ-linolenic acid (GLA, 18: 3n−6) and arachidonic acid (ARA, 20: 4n−6); and ω3 unsaturated fatty acids such as α-linolenic acid (ALA, 18: 3n−3), eicosatetraenoic acid (ETA, 20: 4n−3), eicosapentaenoic acid (EPA, 20: 5n−3) and docosahexaenoic acid (DHA, 22: 6n−3). The highly-unsaturated fatty acids are not only involved in regulating fluidity of membrane as a major constituent of biological membrane but also important as a precursor of a biofunctional component. ARA and EPA serve as precursors of e.g., prostaglandin, thromboxane and leukotriene in higher animals; whereas DHA is a highly-unsaturated fatty acid present most abundantly in the brain. EPA has physiological effects, such as a platelet aggregation inhibitory effect, a blood triglyceride lowering effect, an anti-arteriosclerotic effect, a blood viscosity-lowering effect, a blood pressure-lowering effect, an anti-inflammatory effect, an anti-tumor effect and used in various fields including pharmaceuticals, foods, cosmetics and animal feeds. Recently, in view of lifestyle-related disease prevention, active intake of ω3 unsaturated fatty acids is recommended. Likewise, ω3 unsaturated fatty acids are lipid molecular species significantly increased in demand.
DHA and EPA of living bodies are not only taken from food but also biosynthesized from ALA in some organisms. However, since ALA cannot be biosynthesized in humans, DHA and EPA are nutritionally essential fatty acids for humans. EPA is abundantly contained mainly in oils of fish such as cod, herring, mackerel, salmon, sardine and krill; psychrotrophic marine bacteria such as Shewanella livingstonensis; and algae such as Labyrinthulomycetes. Methods for extracting or purifying EPA from these biological resources have been known. The most common method is purification of EPA from fish oil. However, the EPA content in fish oil is low. In addition to this problem, depending upon the extraction or purification method, fish odor sometimes remains in EPA derived from fish oil, and the content of erucic acid, which is said as a cause a heart disease, increases.
Recently, oleaginous microorganisms accumulating lipid within cells, have drawn attention in connection with energy problems and methods for microbiologically producing various types of lipids have been developed. For example, studies on a method for producing a highly-unsaturated fatty acid using microorganisms of a filamentous fungus belonging to the genus Mortierella have been conducted. Mortierella microorganisms are known to have an ω3 or ω6 highly unsaturated fatty acid metabolic pathway to produce EPA (Non Patent Literature 1). Patent Literature 1 discloses a method for producing EPA by culturing an EPA-producing Mortierella microorganism. Patent Literature 2 discloses a method for producing ARA and EPA using a mutant strain obtained by subjecting Mortierella alpina to a mutation treatment. Patent Literature 3 discloses a method for producing a highly-unsaturated fatty acid such as EPA by using a transformed strain obtained by introducing a gene encoding an ω3 unsaturated polypeptide isolated from Mortierella alpina into a yeast.
However, the ω3 desaturase of a Mortierella microorganism has a low optimum temperature and does not sufficiently function at normal temperature (about 20° C.) where the microorganism easily proliferates. For the reason, even if the Mortierella microorganism is cultured at normal culture temperature, EPA cannot be efficiently produced. In addition, since the ω3 desaturase of the Mortierella microorganism preferentially acts on a fatty acid having 18 carbon atoms, it was difficult to efficiently produce EPA having 20 carbon atoms by the conventional method using the Mortierella microorganism.
In the circumstances, it has been desired to develop an ω3 desaturase capable of efficiently synthesizing EPA from a fatty acid having 20 carbon atoms (e.g., ARA). An ω3 desaturase isolated from Saprolegnia diclina is disclosed in Patent Literature 4; Δ17 desaturase isolated from Phytophthora ramorum in Patent Literature 5; and Δ17 desaturase isolated from Pythium aphanidermatum in Patent Literature 6.